Hydraulic fracturing is a means of creating fractures emanating from the well bore in a producing formation to provide increased flow channels for production. A viscous fluid containing a proppant such as sand is injected under high pressure until the desired fracturing is achieved. The pressure is then released allowing the fluid to return to the well. The proppant, however, remains in the fractures preventing them from closing.
Proppants are particulates that resist high temperature, pressure, and closure stresses present in the formation. If proppants fail to withstand the closure stresses of the formation, they disintegrate to produce fines or fragments, which reduce the permeability of the propped fracture.
In the prior art, proppants include silica sand, glass beads, walnut shells, and aluminum microspheres. Based on a balance of cost and compressive strength, silica sand, commonly known as frac-sand, is a widely used proppant in fracturing. However, its use is limited to a depth with closure stresses of about 41 MPa (Mega Pascal). One MPa equals one million Pascals. One Pascal equals the force of one Newton per square meter area. Beyond this depth resin-coated and ceramic proppants are used. Resin-coated and ceramic proppants are generally limited to closure stresses of about 55 MPa and 83 MPa, respectively.
According to a study for the United States Department of Energy, published in April 1982 (Cutler and Jones, ‘Lightweight Proppants for Deep Gas Well Stimulation’ DOE/BC/10038-22), ideal proppants for hydraulic fracturing should have a specific gravity less than 2.0, be able to withstand closure stresses of 138 MPa, be chemically inert in brine at temperatures up to 200° C., have perfect sphericity, cost the same as sand on a volume basis, and have a narrow proppant size distribution. The report concludes that such a proppant is not likely to be forthcoming in the foreseeable future.